UNIVERSITY    OF   CALIFORNIA 

COLLEGE    OF    AGRICULTURE 

AGRICULTURAL   EXPERIMENT  STATION 

CIRCLUAR  No.  235 

April,  1922 

SOIL  ANALYSIS  AND  SOIL  AND  PLANT 
INTERRELATIONS 

By  D.  E.  HOAGLANDi 


There  exists  a  common  impression  that  from  a  chemical  analysis 
of  a  sample  of  soil  reliable  indications  may  be  obtained  concerning 
the  productivity,  fertilizer  requirements,  or  crop  adaptation  of  the 
area  which  the  sample  is  supposed  to  represent.  It  is  the  purpose  of 
this  publication  to  show  why  this  idea  is  erroneous  and  to  describe  in 
brief  outline  certain  modern  views  on  the  interrelations  of  soils  and 
plants. 

It  is  true  that  Hilgard,  as  well  as  other  soil  chemists,  was  inclined 
to  attach  considerable  importance  to  soil  analysis  as  applied  to  Cali- 
fornia soils,  but  within  recent  years  investigation  on  soils  and  plants 
have  progressed,  and  many  of  the  older  ideas  with  regard  to  their 
relations  are  being  discarded  as  a  result  of  more  recent  researches. 
In  this  respect  soil  and  plant  investigations  do  not  differ  from  those 
in  any  other  field  of  science.  New  methods  of  experimentation,  new 
ideas  gained  from  other  sciences,  new  opportunities  for  critical  study, 
all  enable  the  present-day  student  of  plant  nutrition  to  obtain  a  much 
more  accurate  conception  of  the  interrelationships  of  soils  and  plants 
than  was  possible  in  the  past. 

According  to  the  older  teachings,  which  are  still  promulgated  to  a 
considerable  extent,  it  is  very  important  to  ascertain  the  total  amounts 
of  potassium,  nitrogen,  phosphorus,  calcium,  etc.,  contained  in  an  acre- 
foot  of  soil,  or  if  not  the  total  quantities  then  the  percentages  of  the 
important  elements  soluble  in  some  acid,  such  as  hydrochloric  acid,  in 
Hilgard 's  method  of  soil  examination.  If  the  analysis  of  a  soil  showed 
less  than  a  certain  percentage  (a  varying  standard)  of  one  of  the 
principal  nutrient  elements  of  the  soil,  then  it  was  frequently  thought 
that  the  soil  would  respond  to  applications  of  fertilizers  containing 
that  element.  Another  modification  of  this  view  is  to  the  effect  that 
it  is  essential  to  determine  the  total  quantities  of  important  elements 

i  Acknowledgment  is  made  of  the  review  of  the  manuscript  by  Professors 
Burd,  Hibbard,  Kelley,  and  Lipman,  of  the  College  of  Agriculture. 


2  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

present  in  the  soil  as  a  sort  of  inventory,  the  results  of  which  are 
thought  to  indicate  the  probable  duration  of  the  fertility  of  the  soil. 
The  assumption  is  also  made  sometimes  that  high  totals  imply  corre- 
spondingly large  quantities  of  plant  foods  available  to  the  plant. 
According  to  still  other  teachings,  it  is  urged  that  plant  foods2  must 
be  restored  to  the  soil  in  proportion  to  withdrawals  by  the  crop. 

In  discussing  these  various  points  of  view  it  may  be  convenient  to 
describe  first  certain  experiments  carried  on  during  the  past  eight  years 
by  the  Division  of  Agricultural  Chemistry  (now  part  of  the  Division 
of  Plant  Nutrition).  At  the  beginning  of  the  experiments  large  quan- 
tities of  different  soils  collected  in  various  widely  separated  sections 
of  the  state  were  assembled  in  Berkeley.  Seven  of  these  soils  were 
classified  as  fine  sandy  loams  of  the  same  soil  series  and  seven  as  silty 
clay  loams  of  different  soil  series.  A  set  of  tanks,  each  of  about  one- 
ton  capacity,  was  installed,  and  after  very  thorough  sieving  and  mixing 
two  tanks  were  filled  with  each  soil,  twenty-eight  tanks  in  all.  (One 
soil  was  found  to  produce  practically  no  crop  and  is  not  considered 
in  the  present  discussion.)  During  the  first  season  all  the  tanks  were 
cropped  with  barley.  In  all  subsequent  seasons  only  one  tank  of  each 
soil  was  planted,  the  others  remaining  uncropped,  but  otherwise  treated 
similarly.  Great  care  was  taken  to  maintain  uniform  and  favorable 
moisture  conditions  in  all  tanks  during  the  growing  season.  A  repre- 
sentative sample  of  each  soil  was  analyzed  for  the  total  percentages 
of  all  the  important  elements  present.  Determinations  were  also  made 
of  the  amounts  of  these  elements  soluble  in  strong  hydrochloric  acid 
(Hilgard's  method)  and  in  citric  acid  (a  weak  acid). 

One  important  part  of  the  investigation  consisted  in  making  ex- 
tracts with  pure  water  of  large  samples  of  soil  taken  from  the  different 
tanks  and  then  analyzing  these  extracts.  The  analyses  were  made  not 
on  one  sample  merely  but  samples  were  taken  every  two  weeks  during 
the  growing  season  and  analyzed  in  the  same  way. 

At  the  present  time,  after  seven  or  eight  years  of  intensive  and 
laborious  work  on  these  soils  (many  other  experiments  were  performed 
which  are  too  technical  for  this  discussion),  certain  general  principles 
have  been  suggested,  which  also  receive  support  from  a  number  of 
other  investigations  and  from  theoretical  considerations.  Using  the 
results  of  the  California  experiments  by  way  of  illustration,  we  shall 
now  make  a  further  inquiry  into  some  of  the  common  conceptions  of 
the  soil  in  its  relation  to  the  plant.     In  the  first  place,  the  investiga- 


2  This  commonly  used  expression  is  not  scientific;) I ly  accurate,  but  is  retained 
Tor  convenience.  It  is  used  to  denote  certain  chemical  elements  in  the  soil  which 
are  essential  to  plant  growth,  especially  nitrogen,  phosphorus,  and  potassium. 


Circular  235]      sqiL  analysis  and  plant  interrelations  3 

tions  conducted  in  Berkeley  show  that  there  is  no  consistent  relation 
whatever  between  the  crop  yield  in  any  year  and  the  total  quantity 
of  any  element  present  in  the  soil.  Neither  was  such  a  relation  ob- 
served when  the  percentages  of  the  plant  food  elements  soluble  in 
hydrochloric  acid  were  considered.  The  data  for  the  citric  acid  ex- 
tracts were  only  slightly  more  significant.  On  the  other  hand,  careful 
study  of  the  amounts  of  plant  foods  (especially  nitrate)  extracted  by 
water  showed  certain  interesting  relations  to  crop  yields.  No  one 
observation  had  any  particular  significance,  but  it  was  necessary  to 
take  into  consideration  the  data  obtained  on  water  extracts  of  samples 
of  soil  taken  during  various  periods  of  crop  growth. 

It  is  believed  that  these  and  other  recent  experiments  emphasize  a. 
number  of  important  principles  involved  in  the  interrelations  of  soils 
and  plants.  The  plant  absorbs  mineral  nutrients  from  the  soil  solu- 
tion.3 Only  after  the  required  elements  are  dissolved  in  this  solution 
do  they  become  of  use  to  the  plant.  Now  the  amounts  of  the  plant 
foods  found  in  the  soil  solution  at  any  time  are  exceedingly  small  when 
compared  with  the  total  quantities  contained  in  the  soil.  Moreover, 
the  composition  of  the  soil  solution  is  by  no  means  constant.  In  fact, 
one  of  the  outstanding  characteristics  of  such  a  solution  is  that  it  is 
changing  even  from  day  to  day.  For  example,  in  the  case  of  nitrate, 
one  of  the  most  variable  constituents,  at  the  beginning  of  the  growing- 
season  comparatively  large  amounts  may  be  found  in  the  soil  mois- 
ture, but  after  the  crop  has  grown  for  some  weeks  or  months  scarcely 
any  nitrate  may  remain.  This  is  equally  true  of  both  a  very  produc- 
tive and  a  slightly  productive  soil.  The  quantity  of  water-soluble 
potassium,  calcium,  magnesium,  and  other  elements  may  also  be  mark- 
edly decreased  at  this  period.  In  many  cases  the  total  concentration 
of  plant  foods  dissolved  in  the  soil  solution  may  be  very  slight  at  the 
time  when  the  crop  is  making  its  greatest  draughts.  Not  only  does 
the  composition  of  the  soil  solution  affect  the  growth  of  the  plant,  but 
the  growth  of  the  plant  reciprocally  affects  the  soil  solution.  In- 
creased quantities  of  certain  elements  may  be  brought  into  solution  as 
a  result  of  absorption  by  the  plant.  For  this  reason  the  total  quantity 
of  phosphorus  found  in  a  mature  plant  may  be  very  much  greater 
than  the  amount  present  in  the  soil  solution  at  any  one  time. 

These  and  many  other  facts  have  led  to  the  conclusion  that  the 
really  important  consideration  is  the  availability  of  plant  foods  at 
the  proper  stages  of  plant  growth.  In  other  words,  at  those  periods 
when  the  plant  needs  to  draw  most  heavily  an  the  nutrients  adequate 

3  The  term   "  soil   solution"  is  used  here  to  signify   the   soil   moisture   from 
which  the  plant  can  absorb  dissolved  substances. 


4  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

supplies  must  already  be  dissolved  in  the  soil  solution,  or  be  capable  of 
entering  into  solution  with  sufficient  rapidity.  Thus  the  rate  at  which 
the  soil  solution  is  replenished  with  plant  foods  as  these  are  withdrawn 
by  the  plant  is  the  vital  question.  Unless  we  have  some  assurance  that 
these  processes  take  place  in  such  a  manner  as  to  meet  the  requirements 
of  the  plant,  it  is  of  little  interest  to  determine  the  total  amounts  of 
the  plant  food  elements  present  in  the  whole  mass  of  soil. 

Keferring  again  to  the  California  experiments,  it  was  found  that 
in  these  soils  the  total  amounts  of  nitrogen  in  an  acre-foot  would  be 
equivalent  to  the  amounts  removed  by  large  barley  crops  over  a  period 
of  50  to  100  years,  for  potassium  from  1000  to  several  thousand  years, 
and  for  phosphorus  several  hundred  years.  Yet  the  actual  crop  yields 
from  year  to  year  showed  that  some  of  the  soils  were  only  one-third 
as  productive  as  others,  and,  as  we  have  stated,  the  differences  bore  no 
consistent  relation  to  the  total  amounts  of  plant  foods  present,  nor  to 
the  amounts  soluble  in  acids.  After  seven  years'  cropping,  several 
soils  showed  a  decided  decline  in  yield,  while  others  maintained  their 
original  production  to  a  considerable  degree.  These  changes  also  were 
independent  of  the  total  or  the  acid-soluble  plant  foods.  To  illustrate, 
after  seven  years'  cropping,  one  soil  produced  a  crop  nearly  three 
times  as  large  as  that  yielded  by  another  soil  of  the  same  physical 
character  and  originally  containing  an  identical  percentage  of  total 
nitrogen  and  similar  percentages  of  other  elements.  Since  in  these 
experiments  moisture  and  physical  conditions  were  made  as  favorable 
and  uniform  as  possible,  every  opportunity  was  afforded  to  bring  out 
relations  between  the  analyses  of  the  soil  and  crop  production,  if  such 
relations  existed. 

It  may,  of  course,  be  claimed  that  the  total  amount  of  any  element 
present  in  the  soil  represents  a  potential  supply,  which  may  be  made 
available  by  suitable  management.  But  of  what  value  is  it  to  learn 
that  one  soil  contains  in  an  acre-foot  a  total  supply  of  potassium  suffi- 
cient for  one  thousand  crops  and  another  soil  a  supply  for  two  thou- 
sand crops  when  the  information  gives  no  clue  to  the  availability  of 
this  potassium  during  the  next  year  or  the  next  fifty  years  ?  It  is  very 
essential  to  learn  just  how  the  supply  of  potassium  in  the  soil  can  be 
made  to  dissolve  in  the  soil  solution  at  such  rates  or  times  as  to  meet 
the  requirements  of  the  plant.  No  hint  of  the  way  in  which  this  con- 
dition can  be  brought  about  is  contained  in  the  figures  indicating  the 
total  quantity  of  potassium  in  the  soil.  Similar  reasoning  applies  to 
all  the  plant  foods. 

It  is  true  that  this  series  of  thirteen  soils  did  not  include  any  soil 
of  extremely  low  productivity.    It  is  certain  that  many  highly  infertile 


Circular  235]      soil,  analysis  and  plant  interrelations  5 

soils  would  also  show  a  very  low  content  of  one  or  all  of  the  important 
plant  foods.  Such  soils,  however,  are  generally  recognized  without 
any  need  of  a  chemical  analysis.  In  California,  at  least,  soils  of  this 
character  are  seldom  submitted  to  the  chemist  for  examination.  It 
appears  that  many  of  the  older  ideas  concerning  the  general  utility  of 
soil  analysis  were  based  on  comparisons  of  exceptionally  poor  soils  and 
highly  productive  soils. 

If  a  soil  analysis  gives  no  indication  whether  or  not  plant  foods 
can  be  made  available  to  the  plant  at  the  proper  times,  it  necessarily 
follows  that  such  an  analysis  cannot  be  used  to  determine  what  method 
of  fertilization  should  be  employed.  When  any  soluble  material  is 
added  to  a  soil  exceedingly  complex  chemical  and  biological  changes 
take  place.  The  addition  of  one  element  may  increase  the  availability 
of  other  elements.  For  example,  the  addition  of  calcium  to  some  soils 
may  increase  the  quantity  of  potassium  in  the  soil  solution.  These 
alterations  in  the  chemical  composition  of  the  soil  moisture  may  also 
affect  the  life  processes  of  the  soil  microorganisms,  perhaps  changing 
the  rate  at  which  organic  matter  is  broken  down,  or  nitrates  formed, 
with  very  significant  effects  on  the  yield  or  quality  of  crops. 

The  conclusion  is  also  clear  that  an  estimate  of  the  amounts  of 
plant  food  withdrawn  by  the  crop  furnishes  no  scientific  basis  for  soil 
treatment.  Certain  elements,  such  as  potassium,  may  become  ade- 
quately available  under  proper  conditions  without  any  new  supply 
being  added  to  the  soil,  while  nitrogen  in  some  cases  may  need  to  be 
applied  in  even  greater  quantity  than  would  be  estimated  from  crop 
withdrawals.  The  point  to  be  emphasized  is  that  neither  the  analysis 
of  the  crop  nor  the  analysis  of  the  soil  affords  the  essential  information 
concerning  availability  of  plant  food  at  various  periods,  and  conse- 
quently such  data  cannot  serve  as  a  reliable  guide  in  soil  management. 

Turning  now  to  another  closely  related  phase  of  this  subject,  it  is 
sometimes  claimed  that  different  crops  respond  to  different,  although 
very  definite,  fertilizer  mixtures.  Practically  nothing  is  known  of  the 
exact  requirements  of  any  crop,  but  even  if  such  information  were  at 
hand  fertilizer  mixtures  cannot  be  prescribed,  since  the  application 
of  such  mixtures  is  likely  to  produce  in  different  soils,  or  even  in  the 
same  soil  under  various  treatments,  different  amounts  of  available 
plant  foods,  and  these  effects  are  so  complex  that  they  cannot  be  pre- 
dicted at  the  present  time. 

In  any  discussion  of  the  relations  of  soils  and  plants  reference 
must  be  made  to  the  importance  of  such  factors  as  light,  temperature, 
humidity,  total  soil  moisture,  and  soil  aeration.  If  a  maximum  crop 
is  to  be  produced,  there  must  be  suitable  adjustment  between  all  the 


b  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

factors  involved.  Under  one  set  of  seasonal  influences  a  soil  might 
provide  an  environment  in  the  soil  moisture  highly  favorable  to  a 
certain  plant,  while  under  other  climatic  conditions  the  availability 
of  the  plant  foods  in  the  soil  might  be  less  fortunately  adjusted  to  the 
plant,  resulting  in  a  far  smaller  crop  yield.  Here,  again,  soil  analysis 
lends  no  aid. 

There  remains  still  another  reason  why  soil  analysis,  as  ordinarily 
practiced,  does  not  accomplish  any- valuable  purpose.  Even  if  it  were 
possible  to  make  a  chemical  analysis  of  a  soil  which  would  be  capable 
of  any  consistent  interpretation,  it  would  first  be  necessary  to  secure 
a  sample  truly  representative  of  the  field  or  area  in  question.  Soils 
in  general  are  known  to  be  exceedingly  variable  and  samples  taken 
only  a  few  feet  apart  may  give  very  different  results.  These  conclu- 
sions are  drawn  from  extensive  experiments  initiated  by  the  Division 
of  Soil  Chemistry  and  Bacteriology  (now  part  of  the  Division  of  Plant 
Nutrition).  The  difficulty  of  obtaining  samples  of  soil  adequately 
representing  any  area  is  so  great  that  such  sampling  is  impracticable 
as  a  routine  procedure. 

No  consideration  has  been  given  to  alkali  soils  in  this  circular. 
These  require  separate  treatment,  and  the  reader  is  referred  to  another 
publication1  of  the  Agricultural  Experiment  Station  for  a  discussion 
of  soils  containing  excessive  quantities  of  soluble  salts.  Certain  chem- 
ical tests  are  now  being  made  on  alkali  soils  when  the  samples  are  taken 
under  the  supervision  of  the  farm  advisor. 

Briefly  restating  the  main  point  of  the  present  discussion,  we  may 
conclude  that  the  soil  should  not  be  regarded  simply  as  a  storehouse 
of  plant  foods,  with  a  value  to  be  assessed  according  to  the  total  supply 
of  these  elements  present.  A  more  accurate  analogy  is  that  of  an 
exceedingly  complex  and  constantly  changing  chemical  system  in  which 
the  chemical  processes  of  the  soil  and  of  the  plant  have  an  intimate 
relation.  The  soil  miner als  enter  into  solution  at  a  rate  which  is  de- 
pendent on  many  factors,  such  as  carbon  dioxide  production  by  micro- 
organisms and  by  the  plant,  nitrate  production,  rates  at  which  the 
plant  absorbs  the  various  elements,  temperature,  moisture,  etc.  In 
other  words,  the  growth  of  the  plant  is  determined  by  the  nature  of 
the  chemical  changes  taking  place  in  the  soil  and  plant,  and  not  by 
the  percentage  composition  of  the  soil.  No  method  of  analysis  appli- 
cable to  individual  samples  of  soil  can  yield  the  kind  of  information 
which  it  is  essential  to  possess  in  order  to  draw  any  useful  or  reliable4 
conclusion  concerning  plant  growth. 


i  The  Present  Status  of  Alkali,  by  W.  P.  Kelley,  Circular  219. 


Circular  235]       S0IL  ANALYSIS  AND  PLANT  INTERRELATIONS  7 

Notwithstanding  the  exceedingly  difficult  nature  of  the  problem, 
progress  has  been  made  in  the  field  of  plant  nutrition.  The  hope  of 
a  better  understanding  of  the  principles  of  plant  growth  and  of  the 
interrelations  of  soils  and  plants  depends  upon  continued  scientific 
investigation.  By  this  is  meant  much  more  than  field  trials  of  various 
fertilizers.  A  real  comprehension  of  the  nature  of  the  soil  as  a  medium 
for  plant  growth  must  be  based  on  a  knowledge  of  the  laws  of  chemistry 
and  physics  and  biology  as  applied  to  the  soil  and  to  the  plant. 

Meanwhile  practical  advice  concerning  specific  problems  in  the 
management  of  soils  must  in  general  come  from  the  farm  advisor. 
Because  of  the  many  factors,  both  agricultural  and  economic,  which 
must  be  taken  into  consideration,  a  knowledge  of  local  conditions  is 
ordinarily  indispensable  to  any  plan  for  soil  treatment. 


